The field of the invention is magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to a method and means for producing magnetic field gradients in MRI systems.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field along the z-axis. Additionally, there is a wobbling or precession occurring about this magnetic field, the rate of precession being the Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x, G.sub.y, and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The magnetic field gradient subsystem of an MRI system is perhaps the most critical subsystem in defining the utility of a scanner. In general, more powerful gradient subsystems give greater applications capability. The power of a gradient subsystem is roughly equivalent to the product of the gradient amplitude and the gradient slew-rate. Gradient amplitude is determined by the current which the gradient amplifiers produce in the gradient coils, and gradient slew rate is the rate at which the gradient amplifiers can change the gradient amplitude.
In many circumstances, the only factor of importance in the generation of a gradient field pulse is the integral of gradient amplitude over the duration of the gradient pulse (i.e. the gradient pulse area). This is true, for example, with slice-select refocusing, phase-encoding, velocity or flow compensation, spoiling, rewinding and readout defocusing gradient pulses. Since the shortest duration gradient pulse of a given area provides the greatest flexibility in selecting pulse sequence echo time (TE) and pulse sequence repetition time (TR), it is highly desirable for the MRI system to produce these gradient pulses with the minimum pulse duration possible given the prescribed pulse area.
There is a need to provide minimum duration gradient pulses that do not operate beyond the physiological limits, for example as established by the Reilly equation.